Heterosis: Performance Stability, Adaptability to Changing Technology, and the Foundation of Agriculture as a Business
The agriculture of the past century has been romanticized as an ideal of a simple, virtuous, and good life, and is a cornerstone of deeply held beliefs in America. Considered as a business, this agriculture featured small farms, labor intensive inputs (both human and animal) and minimal mechanical, chemical, and capital inputs. Hybrids were uniquely positioned to take advantage of many of the changes that occurred in the business of agriculture in this century: increased chemical inputs, mechanization, capitalization, farm size and decreased use of human and animal labor. Heterosis allowed more effective use of the available genetic resources in open-pollinated varieties. Interest in heterosis, specifically maize hybrids, also laid the foundation for private and public research, which fostered the continued development of improved maize hybrids. Yield stability, in addition to increased yield potential, were important stimulants to the capital investment in farming. During the normal years in the 1920s and 1930s when open-pollinated varieties predominated, about 85% of the maize (Zea mays L.) planted was harvested. During the drought stress years of 1934 and 1936 harvest rates dipped to only 61 and 67%. After widespread adoption of hybrids (1940 and beyond), the proportion of harvested hectares has fluctuated from 85 to 92%, regardless of the environment of any particular year; however, the yield in each of these high stress years was lower than that recorded in more normal years. Modem hybrid maize differs primarily from open-pollinated and earlier hybrids in their response to stress. Newer hybrids have improved water stress performance, are much less prone to silk delay, have significantly lower respiration rates during silking, have longer periods of grain fill, and are higher yielding under both low and high input environments. Tassel size has decreased as has premature death (stay green scores are improved) and stalk rot has decreased. Resistance to European Com Borer 2nd brood has increased. Hybrids in other species, such as tomatoes (Lycopersicon esculentum L.), demonstrate other attributes that may have been of equal importance to adoption in agriculture. The two parents of a hybrid may combine unique combinations of traits not available in either single parent. Increased shelf life in fresh market tomatoes is one such example. The longer shelf life tomato may be analogous to yield stability in maize since harvest can occur over a longer time window within a given environment. Yield stability and better predictability have profound implications: Geographic areas that favor higher yields and returns will displace those areas in which yields are lower (total hectares for maize in the USA are lower today than in 1920). Business unit size (i.e., farm size) will increase as long as profitability within a commodity is higher. Choice of hybrids by farmers dictate both the scale of research and by whom research continues in hybrids, i.e., companies with larger market share tend to have more research.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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